The girl and her cousin had reportedly gone wandering off during the family barbecue and adult members of the family didn't know.

Via the New York Post:

The body of Ruhshona Kurbonova was pulled from the water by NYPD divers at about 6 p.m., some five hours after she and her 3-year-old cousin were ­reported missing.

The body of Ruhshona Kurbonova was found in the water by the NYPD. One of the dozens of park-goers who joined in the desperate search for the kids found the little boy, soaked and covered in algae, in a wooded area. He was unhurt.

The toddler's mother was reportedly taken away on a stretcher crying after learning that NYPD divers had discovered the child's body. A collaborative rescue effort had spread like wildfire through Prospect Park once it was learned that Ruhshona had gone missing.